In telephony, there exists the problem that payload data (for example, acoustic signals such as voice signals) and signalling data, such as information concerning receiver status, digital signals entered via the keypad, ringing signals, etc., must be transmitted over the same channel or the same information transmission medium. Current telephony systems use a pulse code-modulated (PCM) channel, i.e. all data is transmitted digitally. Such a PCM transmission method is described, for example, in the textbook "Traite d'Electricit," Volume XVIII of "Syste mes de Telecommunications" by Pierre-Gerard Fontolliet, published by Ecole Polytechnique Federal de Lausanne. Both payload data and signalling data, which may occur simultaneously, must be transmitted using the same single transmission medium.
There are currently two predominant standards for transmitting signalling data over a standard PCM channel: In the European PCM basic system (ITU-T G.732), the signalling data of 30 telephone channels are multiplexed time slots (off-band signalling). This system is known as "Channel Associated Signalling" (CAS). According to the American PCM basic system (ITU-T G.733), signalling data for 24 telephone channels are distributed using the "Bit-Stealing" method over all 24 channels. But both of the above described methods can be used only in multichannel systems. It would be desirable to be able to transmit signalling data and payload data, with relatively low distortion and bandwidth loss, over any single channel.